We will investigate the role of protein phosphorylation in mechanisms of viral transformation and cell growth control. In recent years we have focused on retroviruses whose transforming proteins have tyrosine-specific protein kinase activity. We have identified a number of substrates for tyrosine specific kinases, and we plan to extend our characterization of these substrates and the genes which encode them. We have begun to study growth factor receptors with inducible tyrosine protein kinase activities. We will expand our characterization of these receptors and their relationship to viral transforming proteins. These studies will provide insight into molecular mechanisms of growth control related to cancer.